The possibilities of the integral equation (IE) approach to study the mixed-gas adsorption equilibria and accompanying enthalpic effects are presented. As the model of energetic heterogeneity the rectangular adsorption energy distribution functions were used. Also the justification of the use of the condensation approximation (CA) in that case was discussed. The theoretical expressions are in good conformity with experimental data. The presented theoretical expressions for mixed-gas adsorption equilibria can explain the azeotropic behaviour. (C) 2004 Elsevier B.V. All rights reserved.